1. Field of the Invention
This invention relates to microprocessors and, more particularly, to decoding variable-length instructions within a microprocessor.
2. Description of the Relevant Art
The number of software applications written for the x86 instruction set is quite large. As a result, despite the introduction of newer and more advanced instruction sets, microprocessor designers have continued to design microprocessors capable of executing the x86 instruction set.
The x86 instruction set is relatively complex and is characterized by a plurality of variable-length instructions. A generic format illustrative of the x86 instruction set is shown in FIG. 1. As illustrated in the figure, an x86 instruction consists of from one to five optional prefix bytes 102, followed by an operation code (opcode) field 104, an optional addressing mode (Mod R/M) byte 106, an optional scale-index-base (SIB) byte 108, an optional displacement field 110, and an optional immediate data field 112.
The opcode field 104 defines the basic operation for a particular instruction. The default operation of a particular opcode may be modified by one or more prefix bytes 102. For example, one of prefix bytes 102 may be used to change the address or operand size for an instruction, to override the default segment used in memory addressing, or to instruct the processor to repeat a string operation a number of times. The opcode field 104 follows prefix bytes 102, if present, and may be one or two bytes in length. The addressing mode (Mod R/M) byte 106 specifies the registers used as well as memory addressing modes. The scale-index-base (SIB) byte 108 is used only in 32-bit base-relative addressing using scale and index factors. A base field within SIB byte 108 specifies which register contains the base value for the address calculation, and an index field within SIB byte 108 specifies which register contains the index value. A scale field within SIB byte 108 specifies the power of two by which the index value will be multiplied before being added, along with any displacement, to the base value. The next instruction field is a displacement field 110, which is optional and may be from one to four bytes in length. Displacement field 110 contains a constant used in address calculations. The optional immediate field 112, which may also be from one to four bytes in length, contains a constant used as an instruction operand. The shortest x86 instructions are only one byte long and comprise a single opcode byte. The 80286 sets a maximum length for an instruction at 10 bytes, while the 80386 and 80486 both allow instruction lengths of up to 15 bytes.
The complexity of the x86 instruction set poses many difficulties in implementing high performance x86-compatible microprocessors. In particular, the variable length of x86 instructions makes decoding instructions difficult. Decoding instructions typically involves determining the boundaries of an instruction and then identifying each field within the instruction, e.g., the opcode and operand fields. Decoding typically takes place when the instruction is fetched from the instruction cache prior to execution.
One method for determining the boundaries of instructions involves generating and storing one or more predecode bits for each instruction byte as it is read from main memory and stored into the instruction cache. The predecode bits provide information about the instruction byte they are associated with. For example, an asserted predecode start bit indicates that the associated instruction byte is the first byte of an instruction. Once a start bit for a particular instruction byte is calculated, it is stored together with the instruction byte in the instruction cache. When a "fetch" is performed, a number of instruction bytes are read from the instruction cache and decoded in preparation for execution. Any associated start bits are scanned to generate valid masks for the individual instructions with the fetch. A valid mask is a series of bits in which each bit corresponds to a particular instruction byte. Valid mask bits associated with the first byte of an instruction, the last byte of the instruction, and all bytes in between the first and last bytes of the instruction are asserted. All other valid mask bits are not asserted. Once the valid mask has been calculated, it may be used to mask off bytes from other instructions.
Turning now to FIG. 2, an exemplary valid mask is shown. The figure illustrates a portion of a fetch 120 and its associated start bits 122. Assuming a valid mask 126 for instruction B 128 is to be generated, start bit 122A, and all bits between start bit 122A and start bit 122B are asserted to generate mask 126. Once generated, valid mask 126 may then be used to mask off all bytes within fetch 120 that are not part of instruction B 128.
As the description above indicates, predecode information may be particularly useful in reducing decode times. By storing the predecode information along with the instruction bytes in the instruction cache, the predecode information need only be calculated once, even if the corresponding instruction is executed a number of time (e.g., in a loop). Unfortunately, however, when the instruction is replaced or discarded from the instruction cache, any associated predecode information is lost. The next time the instruction is read into the instruction cache, predecode information must once again be generated. The time delay caused by waiting for the predecode information to be calculated may be particularly damaging to performance when the instruction is read into the instruction cache as the result of a branch misprediction or a cache miss. In contrast to instructions that are speculatively prefetched before they are needed, fetches resulting from branch mispredictions or cache misses may cause the microprocessor's decoders and functional units to stall while waiting to receive the requested instructions. In this case, the time required to produce the predecode information may have a significant impact upon the performance of the microprocessor.
For these and other reasons, a method and apparatus for reducing predecode times is desired. In particular, a method and apparatus for decreasing the time required to generate predecode information for instructions that have previously been discarded from an instruction cache is desired.